1. Technical Field of the Invention
The present invention relates generally to image-recording apparatus such as a printer or a copy machine and more particularly, for detecting and evaluating coatings on a receiver sheet, which coatings are difficult to detect and evaluate by visual observation.
2. Background of the Invention
It is extremely important that the receiver (transparency or reflection media like receiver) gets loaded with the correct surface facing the proper direction in an apparatus such as a thermal printer or electrophotographic printer that uses a thermoplastic-coated receiver. If the receiver is fed with the wrong side facing the process, the thermoplastic layer will stick to the surface of the transfer drum and the drum must then be cleaned thoroughly and may even have to be changed.
U.S. Pat. No. 5,081,484, issued to Nakata et al on Jan. 14, 1992 discloses a photographic copier that uses specially coated receiver as a receiver. A photosensitive material is exposed and then put in contact with this receiver under pressure. Exposed photocells or microcapsules, as they are sometimes called, are transferred to the receiver, while unexposed cells remain on the photosensitive material. To insure that the receiver is inserted with the correct side of the specially coated receiver facing the photosensitive material, another coating is made on one end of the receiver and only on one side. This extra coating exhibits a particular electrical resistance. Conductive arms in the path of the receiver will read electrical resistance across the receiver as it passes under these arms. When the special-electrical-resistance coating portion of the receiver passes under these arms, a predetermined amount of conductivity is read. This indicates that the receiver was inserted correctly. However, if the predetermined amount of conductivity is not read, a flag is set indicating incorrect receiver loading or improper receiver type. The extra costs of coating a special-electrical-resistance layer is a significant drawback associated with this invention. Another potential problems exists and that is because the coating of the special-electrical-resistance layer is a separate step in addition to the coating of the photo-layer, and coating the special-electrical-resistance layer on the wrong side or the wrong corner of the receiver is a clear possibility.
Another method of surface detection suggested by U.S. Pat. No. 5,081,484 teaches coating a dedicated non-imaging corner of the receiver with dyes. The dyes of a particular color will be used to make these markings. Because the color of the dye is visible, the correct surface of the receiver can be identified by the operator. Also, the correct surface can be identified by the machine if the optical sensors aimed at the receiver surface are tuned for the particular wavelength, i.e., color, of the visible dye. The major disadvantage of this method is the extra visible color markings that are not part of the printed image. The extra receiver surface area used for these markings cannot be used for imaging and is, therefore, wasted. The fact that an additional manufacturing step must be taken to coat the marks with dye is another disadvantage.
U.S. Pat. No. 4,591,271, issued to Byers on May 27, 1986 shows an apparatus that detects coating-side versus no coating-side of any material (transparent or opaque), by applying a puff of condensable vapor to both sides. The coated side absorbs the vapor and does not condense, while the non-coated surface will condense the vapor and cause some fogging on that side. By shining a light to both sides, one can detect which side contains the special coating. The fog on the non-coated side will absorb most of the incident light and a small amount of light is reflected. The coated side, however, will reflect most of the incident light. The reflected light from both sides are compared to determine the coated side versus the non-coated side. This technique applies only to materials that do not contain any coating on one side. Usually, both sides of the receiver sheets are coated with different coatings. The front is coated with the image-sensitive material and the back is covered with a plain resin coating to extend the receiver life and increase its stability as the humidity changes. Therefore, both sides of a normal imaging-type receiver will absorb the condensable vapor and not fog. This can present a problem when using this technique. Furthermore, the act of blowing vapor onto imaging receivers just before imaging takes place, will increase the chance of depositing dust particles, specks and other residue on the receiver sheet. Accordingly, the dust particles and specks will interfere with the normal imaging process and will result in a defect in the final image.
U.S. Pat. No. 5,004,928, issued to Suzuki et al on Apr. 2, 1991 discloses an ink-jet printing apparatus that detects the reflectance of both sides of the receiver simultaneously. By comparing these two signals, the apparatus detects which side of the receiver can be exposed with ink and whether it is inserted correctly. The shortcoming associated with this technique is that if the reflectance of both sides of the receiver are similar, it will fail to correctly distinguish the receiver side. Receivers used for photographic applications, as well as thermal printer receiver, are coated on both sides and exhibit similar reflectance. Furthermore, this technique will not work well with different finishes on each of the surfaces of the receiver. A matte finish on the top surface of a receiver will show lower reflectance than the rear of the receiver. However, a glossy finish on the top surface of the receiver will show considerably higher reflectance than the rear of the receiver.